[Integrated] RFR: 8232063: Upgrade gradle to version 6.0

[Kevin Rushforth]

Changeset: 42040c4c
Author:    Kevin Rushforth <kcr at openjdk.org>
Date:      2019-11-18 18:22:51 +0000
URL:       https://git.openjdk.java.net/jfx/commit/42040c4c

8232063: Upgrade gradle to version 6.0

Reviewed-by: arapte, jvos

! build.gradle
! build.properties
! buildSrc/build.gradle
! gradle/legal/gradle.md
! gradle/wrapper/gradle-wrapper.properties